Field of the Invention
The present invention relates to a lead screw device, a lens driver, a lens barrel, and a camera using the lead screw device.
Description of the Related Art
A lead screw device is used as a lens driver which drives a lens of a camera in an optical axis direction. The lead screw device includes a lead screw which rotates about an axis and a nut member which is threadably mounted on the lead screw, and moves in a straight line along with the rotation of the lead screw about an axis. The lead screw device is configured to hold a lens in a lens holder connected to the nut member. A focus adjuster described in JP3766379B, for example, uses such a lead screw device.
In this lead screw device, it is necessary to threadably mount the nut member on the lead screw from one end portion of the lead screw in the case of mounting the nut member on the lead screw. However, in the lead screw device described in JP3766379B, both ends of the lead screw in the axis direction are supported by a U-shaped supporting member. The nut member cannot be therefore assembled to the lead screw. For this reason, it is necessary to assemble the lead screw after assembling the nut member to the lead screw in advance or to provide a special supporting structure of the lead screw such that the nut member can be assembled after assembling the lead screw. Such requirements for the assembling order or the special structure trigger an increase in the manufacturing cost of a lens driver.
As a method of solving the above problem, it is considered to use a half nut described in JP2009-80248A as the nut member. This half nut is a nut member in which a circular nut member is cut in a radial direction. The half nut can be laterally threadably mounted on the lead screw, namely, the lead screw can be threadably mounted on the lead screw from the radial direction even in a state in which the lead screw is assembled. Therefore, the above-described requirements for the assembling order and the special structure are unnecessary, and an increase in the manufacturing cost of the lens driver can be avoided.
With the configuration using the half nut as described in JP2009-80248A, the half nut is threadably mounted on the lead screw only from one side of the lead screw in the radial direction of the lead screw. Thus, the strength of the half nut when threadably mounted on the lead screw is reduced. For this reason, tooth skipping of the half nut easily occurs when an external force in the axis direction of the lead screw is applied to the half nut. More specifically, the half nut moves over the thread of the lead screw by the external force, so that the half nut moves in the axis direction of the lead screw. In order to prevent the tooth skipping, if the half nut is pressed to the lead screw in a radial direction so as to have strong contact with the lead screw, the stress due to the external force applied to the half nut is concentrated in the position where the half nut is threadably mounted on the lead screw, namely, the thread portions of both of the half nut and the lead screw. For this reason, the thread portions may be damaged due to the stress. In a lens driver, which drives the lens group on a subject side among a plurality of lens groups disposed in a lens barrel, a photographer may touch the lens group, and the external force may be applied to the lens group. Therefore, it is required to control or prevent the tooth skipping or damage on the thread portion due to such an external force.